1. Field of the Invention
This invention relates to a process for forming a silver coating on a surface of a vitreous substrate, in particular to the silvering of glass, that is to say the chemical deposition of a coating of silver, using a silvering solution.
Such a metal coating may be deposited pattern-wise to form a decorative article, but the invention has particular reference to glass substrates bearing a continuous reflective coating. The coating may be applied to a substrate of any form, for example to an artistic object, to achieve some desired decorative effect, but it is envisaged that the invention will find greatest use when the coating is applied to a flat glass substrate. The reflective coating may be so thin that it is transparent. Glass panes bearing transparent reflective coatings are useful inter alia as solar screening panels or as low-emissivity (in respect of infra-red radiation) panels. Alternatively, the coating may be fully reflective, thus forming a mirror-coating. Such a process is also used for the formation of silvered glass microbeads (that is to say microbeads carrying a coating of silver), which may for example be incorporated in a plastics material matrix to form a reflective road-marking paint or a conductive plastics material.
2. Description of the Related Art
Conventionally, silver mirrors are produced as follows. The glass is first of all polished and then sensitised, typically using an aqueous solution of SnCl2. After rinsing, the surface of the glass is usually activated by means of an ammoniacal silver nitrate treatment. The silvering solution is then applied in order to form an opaque coating of silver. This silver coating is then covered with a protective layer of copper and then one or more coats of paint in order to produce the finished mirror.
The silver coating does not always adhere sufficiently to the substrate. In the case of certain prior products, it has been observed that the silver coating comes away spontaneously from the glass substrate. This is, for example, the case when silvered microbeads manufactured in a normal manner are incorporated in a plastics matrix.